Privacy is one of the legitimate exceptions. A swimming and sunbathing area that is designated Women Only, and actually is used by biological females only, is a space in which women could enjoy greater privacy than a mixed sex area.
If I understand rightly, there has always been a Mixed Pond, so there was always a 3rd space for transpeople, so it transpeople had access to the same facilities as men and women had.
I don't see how transpeople were being discriminated against, if there was a mixed 3rd space it always was trans inclusive.